During their joint announcement here, Bush called Graham an "extraordinary" person whom he has admired for some time, telling Fox earlier that the endorsement was "meaningful."

"I admire the fact that as a senator he views his job to forge solutions to problems, rather than to try to tear things down," Bush said at the joint announcement.

He added, “I have sought out his advice even though we were both competitors for the Republican nomination for the presidency. And he gave it! He’s consistently done so. Because he’s not doing this for his own ego.”

Graham said he is backing Bush to ensure “the Republican Party picks a nominee who can win in November.”

"Ladies and gentlemen, South Carolina is going to reset this race," Graham said, looking to next month's GOP primary here. "On February the 20, we are going to give Jeb Bush the momentum that he needs and deserves to win the nomination."

He added: "I have concluded without any hesitation, without any doubt, that Jeb Bush is ready on Day 1 to be a commander in chief worthy of the sacrifices of the 1 percent who have been fighting this war. Last night, he said, ‘I will have their backs.’ I believe he will."

Graham acknowledged Bush's debate performance here in the Queen City the night before. "Last night, he did not talk the most, but he made the most sense," he said.

During a news conference following their remarks, Graham was asked about why he chose not to endorse fellow Sen. Marco Rubio.

"I think Marco Rubio will be president of the United States one day,” Graham said. “I think he’s one of the most gifted people I’ve ever met. I like him. But I wasn’t ready to be president at 44. "

Bush joined in to criticize Rubio, his former protege, for his changing stance on immigration, particularly Rubio's dropped support for an immigration reform bill he co-authored.

"Marco cut and run, plain and simple, for whatever reason, there may be legitimate reasons, but he cut and run,” Bush said. “He asked for my support on a bill, and he cut and run. And he cut and run on his colleagues as well.”

The endorsement is not wholly unexpected. Bush, in recent days, had picked up over 20 of Graham’s former backers. At the last debate in which Graham participated, he famously praised another Bush. As he discussed national security, he went on to discuss the Iraq surge.

"The surge worked! It worked! George W. Bush made mistakes, but he did adjust. I blame Obama for ISIL, not Bush,” Graham yelled. “I’m tired of beating on Bush! I miss George W. Bush! I wish he were president right now! We wouldn’t be in this mess!”

The endorsement, though perhaps not a game-changer for Bush, who has stumbled in the polls since last fall, is all a part of the campaign’s push to appeal to South Carolinians, particularly military personnel and veterans in the state.